Method and apparatus for transmitting ack/nack in a wireless communication system based on tdd

ABSTRACT

Provided is a method of transmitting ACK/NACK in a TDD-based wireless communication system. The method includes: receiving M downlink subframes associated with an uplink subframe n in each of two serving cells; determining four candidate resources based on the M downlink subframes received in each of the two serving cells; and transmitting an ACK/NACK response for the M downlink subframes by using one resource selected from the four candidate resources in the uplink subframe n, wherein the two serving cells includes a first serving cell and a second serving cell, and wherein among the four candidate resources, a first resource and a second resource are associated with a PDSCH or a SPS release PDCCH for releasing semi-persistent scheduling received in the first serving cell, and a third resource and a fourth resources are associated with a PDSCH received in the second serving cell.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for transmitting receptionacknowledgement for a hybrid automatic repeat request (HARQ) in a timedivision duplex (TDD)-based wireless communication system.

BACKGROUND ART

Long term evolution (LTE) based on 3^(rd) generation partnership project(3GPP) technical specification (TS) release 8 is a promisingnext-generation mobile communication standard.

As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 8)”, a physical channel of the LTE can be classified into adownlink channel, i.e., a physical downlink shared channel (PDSCH) and aphysical downlink control channel (PDCCH), and an uplink channel, i.e.,a physical uplink shared channel (PUSCH) and a physical uplink controlchannel (PUCCH).

The PUCCH is an uplink control channel used for transmission of anuplink control signal such as a hybrid automatic repeat request (HARQ)positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signal, achannel quality indicator (CQI), and a scheduling request (SR).

Meanwhile, 3GPP LTE-advanced (LTE-A) which is an evolution of 3GPP LTEis under development. Examples of techniques employed in the 3GPP LTE-Ainclude carrier aggregation and multiple input multiple output (MIMO)supporting four or more antenna ports.

The carrier aggregation uses a plurality of component carriers. Thecomponent carrier is defined with a center frequency and a bandwidth.One downlink component carrier or a pair of an uplink component carrierand a downlink component carrier is mapped to one cell. When a userequipment receives a service by using a plurality of downlink componentcarriers, it can be said that the user equipment receives the servicefrom a plurality of serving cells.

A time division duplex (TDD) system uses the same frequency in downlinkand uplink cases. Therefore, one or more downlink subframes areassociated with an uplink subframe. The ‘association’ implies thattransmission/reception in the downlink subframe is associated withtransmission/reception in the uplink subframe. For example, when atransport block is received in a plurality of downlink subframes, theuser equipment transmits HARQ ACK/NACK for the transport block in theuplink subframe associated with the plurality of downlink subframes.

As a plurality of serving cells are introduced in the TDD system, aninformation amount of HARQ ACK/NACK is increased. Channel selection isone of methods for transmitting the increased HARQ ACK/NACK with alimited transmission bit. The channel selection is a method ofallocating a plurality of radio resources and transmitting a modulatedsymbol by using any one of the plurality of radio resources. A varietyof HARQ ACK/NACK information can be represented according to a signalconstellation of a modulated symbol and a radio resource.

Accordingly, there is a need for a method of allocating a resource toapply such a channel selection to a multiple carrier system supporting aplurality of serving cells.

SUMMARY OF INVENTION Technical Problem

The present invention provides a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) transmission method and apparatusin a time division duplex (TDD)-based wireless communication system.

Technical Solution

According to an aspect of the present invention, a method oftransmitting positive-acknowledgement (ACK)/negative-acknowledgement(NACK) in a time division duplex (TDD)-based wireless communicationsystem in which M (M>2) downlink subframes are associated with an uplinksubframe in each of two serving cells is provided. The method includes:receiving M downlink subframes associated with an uplink subframe n ineach of the two serving cells; determining four candidate resources onthe basis of the M downlink subframes received in each of the twoserving cells; and transmitting an ACK/NACK response for the M downlinksubframes received in each of the two serving cells by using oneresource selected from the four candidate resources in the uplinksubframe n, wherein the two serving cells consist of a first servingcell and a second serving cell, and wherein among the four candidateresources, a first resource and a second resource are related to aphysical downlink shared channel (PDSCH) received in the first servingcell or a semi-persistent scheduling (SPS) release PDCCH for releasingsemi-persistent scheduling, and a third resource and a fourth resourcesare related to a PDSCH received in the second serving cell.

In the aforementioned aspect of the present invention, at least onedownlink subframe among the M downlink subframes received in the firstserving cell may include a PDCCH for transmitting a downlink grant and aphysical downlink shared channel (PDSCH) corresponding to the PDCCH.

In addition, the downlink grant may include a downlink assignment index(DAI) indicating an accumulative counter value of the PDCCH whichtransmits a PDSCH allocated thereto.

In addition, in the M downlink subframes received in the first servingcell, if a PDSCH indicated by detecting a first PDCCH of which a DAIvalue is 1 or a second PDCCH of which a DAI value is 2 is received, orif a first SPS release PDCCH of which a DAI value is 1 or a second SPSrelease PDCCH of which a DAI value is 2 is received, among the fourcandidate resources, the first resource may be determined based on afirst control channel element (CCE) used in transmission of the firstPDCCH or the first SPS release PDCCH, and the second resource may bedetermined based on a first CCE used in the second PDCCH or the secondSPS release PDCCH.

In addition, if an SPS PDSCH not having a corresponding PDCCH isreceived in the M downlink subframes received in the first serving cell,the first resource among the four candidate resources may be oneresource selected from four resources configured by using a higher layersignal, and the selected one resource may be indicated by an uplinktransmit power control field of a PDCCH indicating activation ofsemi-persistent scheduling.

In addition, in the M downlink subframes received in the first servingcell, if a PDSCH indicated by detecting a first PDCCH of which a DAIvalue is 1 or receiving a first SPS release PDCCH of which a DAI valueis 1 is received, or if a first SPS release PDCCH of which a DAI valueis 1 is received, the second resource among the four candidate resourcesmay be determined based on a first CCE used in transmission of the firstPDCCH or the first SPS release PDCCH.

In addition, if a third PDCCH of which a DAI value is 1 and a fourthPDCCH of which a DAI value is 2 is received in the M downlink subframesreceived in the first serving cell, and if a PDSCH indicated bydetecting the third PDCCH or the fourth PDCCH is received in the Mdownlink subframes received in the second serving cell, among the fourcandidate resources, a third resource may be determined based on a firstCCE used in transmission of the third PDCCH, and a fourth resource maybe determined based on a first CCE used in transmission of the fourthPDCCH.

In addition, if at least one PDCCH is received in the M downlinksubframes received in the second serving cell and a PDSCH indicated bydetecting the at least one PDCCH is received in the second serving cell,among the four candidate resources, a third resource and a fourthresource may be selected from four resources configured by using ahigher layer signal, and the selected resources may be indicated by anuplink transmit power control field included in the at least one PDCCH.

Advantageous Effects

The present invention provides a method of transmitting receptionacknowledgement in a time division duplex (TDD) system supporting aplurality of serving cells. Therefore, positive-acknowledgement(ACK)/negative-acknowledgement (NACK) mismatch between a base stationand a user equipment can be decreased.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a downlink radio frame structure in 3^(rd) generationpartnership project (3GPP) long term evolution (LTE).

FIG. 2 shows an example of an uplink subframe in 3GPP LTE.

FIG. 3 shows a physical uplink control channel (PUCCH) format 1b in anormal cyclic prefix (CP) in 3GPP LTE.

FIG. 4 shows an example of performing hybrid automatic repeat request(HARQ).

FIG. 5 shows an example of multiple carriers.

FIG. 6 shows an example of cross-carrier scheduling in a multiplecarrier system.

FIG. 7 shows an example of semi-persistent scheduling (SPS) in 3GPP LTE.

FIG. 8 shows an example of a method of using a bundledpositive-acknowledgement (ACK) counter.

FIG. 9 shows an example of a method of using a consecutive ACK counter.

FIG. 10 shows an ACK/negative-ACK (NACK) resource allocation method incase of cross carrier scheduling.

FIG. 11 shows an example in which an ACK/NACK resource allocation methodis modified in case of cross carrier scheduling.

FIG. 12 shows an example of an ACK/NACK resource allocation method whenthere is SPS physical downlink shared channel (PDSCH) transmission incase of cross carrier scheduling.

FIG. 13 shows an example of resource allocation for channel selectionwhen cross-carrier scheduling is configured.

FIG. 14 shows another example of resource allocation for channelselection when cross carrier scheduling is configured.

FIG. 15 shows an example of a resource allocation method when non-crosscarrier scheduling is configured.

FIG. 16 is a block diagram of a wireless apparatus for implementing anembodiment of the present invention.

MODE FOR INVENTION

A user equipment (UE) may be fixed or mobile, and may be referred to asanother terminology, such as a mobile station (MS), a mobile terminal(MT), a user terminal (UT), a subscriber station (SS), a wirelessdevice, a personal digital assistant (PDA), a wireless modem, a handhelddevice, etc.

A base station (BS) is generally a fixed station that communicates withthe UE and may be referred to as another terminology, such as an evolvednode-B (eNB), a base transceiver system (BTS), an access point, etc.

FIG. 1 shows a downlink radio frame structure in 3^(rd) generationpartnership project (3GPP) long term evolution (LTE). The section 4 of3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial RadioAccess (E-UTRA); Physical Channels and Modulation (Release 8)” may beincorporated herein by reference for time division duplex (TDD).

A radio frame includes 10 subframes indexed with 0 to 9. One subframeincludes 2 consecutive slots. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may have a length of 1 millisecond (ms), and one slot mayhave a length of 0.5 ms.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink(DL), the OFDM symbol is only for expressing one symbol period in thetime domain, and there is no limitation in a multiple access scheme orterminologies. For example, the OFDM symbol may also be referred to asanother terminology such as a single carrier frequency division multipleaccess (SC-FDMA) symbol, a symbol period, etc.

Although it is described that one slot includes 7 OFDM symbols forexample, the number of OFDM symbols included in one slot may varydepending on a length of a cyclic prefix (CP). According to 3GPP TS36.211 V8.7.0, in case of a normal CP, one slot includes 7 OFDM symbols,and in case of an extended CP, one slot includes 6 OFDM symbols.

A resource block (RB) is a resource allocation unit, and includes aplurality of subcarriers in one slot. For example, if one slot includes7 OFDM symbols in a time domain and the RB includes 12 subcarriers in afrequency domain, one RB can include 7×12 resource elements (REs).

A subframe having an index #1 and an index #6 is called a specialsubframe, and includes a downlink pilot time slot (DwPTS), a guardperiod (GP), and an uplink pilot time slot (UpPTS). The DwPTS is used inthe UE for initial cell search, synchronization, or channel estimation.The UpPTS is used in the BS for channel estimation and uplinktransmission synchronization of the UE. The GP is a period for removinginterference which occurs in an uplink due to a multi-path delay of adownlink signal between the uplink and a downlink.

In TDD, a downlink (DL) subframe and an uplink (UL) subframe co-exist inone radio frame. Table 1 shows an example of a configuration of theradio frame.

TABLE 1 Switch- UL-DL point Subframe index configuration periodicity 0 12 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U U D 25 ms D S U D D D S U D D 3 10 ms  D S U U U D D D D D 4 10 ms  D S U U DD D D D D 5 10 ms  D S U D D D D D D D 6 5 ms D S U U U D S U U D

‘D’ denotes a DL subframe, ‘U’ denotes a UL subframe, and ‘S’ denotes aspecial subframe. When the UL-DL configuration is received from the BS,the UE can know whether a specific subframe is the DL subframe or the ULsubframe according to the configuration of the radio frame.

A DL subframe is divided into a control region and a data region in thetime domain. The control region includes up to three preceding OFDMsymbols of a 1st slot in the subframe. However, the number of OFDMsymbols included in the control region may vary. A physical downlinkcontrol channel (PDCCH) is allocated to the control region, and aphysical downlink shared channel (PDSCH) is allocated to the dataregion.

As disclosed in 3GPP TS 36.211 V8.7.0, the 3GPP LTE classifies aphysical channel into a data channel and a control channel. Examples ofthe data channel include a physical downlink shared channel (PDSCH) anda physical uplink shared channel (PUSCH). Examples of the controlchannel include a physical downlink control channel (PDCCH), a physicalcontrol format indicator channel (PCFICH), a physical hybrid-ARQindicator channel (PHICH), and a physical uplink control channel(PUCCH).

The PCFICH transmitted in a 1st OFDM symbol of the subframe carries acontrol format indicator (CFI) regarding the number of OFDM symbols(i.e., a size of the control region) used for transmission of controlchannels in the subframe. The UE first receives the CFI on the PCFICH,and thereafter monitors the PDCCH. Unlike the PDCCH, the PCFICH does notuse blind decoding, and is transmitted by using a fixed PCFICH resourceof the subframe.

The PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink hybridautomatic repeat request (HARQ). The ACK/NACK signal for UL data on aPUSCH transmitted by the UE is transmitted on the PHICH.

A physical broadcast channel (PBCH) is transmitted in first four OFDMsymbols in a 2^(nd) slot of a 1st subframe of a radio frame. The PBCHcarries system information necessary for communication between the UEand the BS. The system information transmitted through the PBCH isreferred to as a master information block (MIB). In comparison thereto,system information transmitted on the PDSCH, indicated by the PDCCH, isreferred to as a system information block (SIB).

The PDCCH is transmitted on an aggregation of one or several consecutivecontrol channel elements (CCEs). The CCE is a logical allocation unitused to provide the PDCCH with a coding rate based on a state of a radiochannel. The CCE corresponds to a plurality of resource element groups(REGs). A format of the PDCCH and the number of bits of the availablePDCCH are determined according to a correlation between the number ofCCEs and the coding rate provided by the CCEs.

Control information transmitted through the PDCCH is referred to asdownlink control information (DCI). The DCI may include resourceallocation of the PDSCH (this is referred to as a DL grant), resourceallocation of a PUSCH (this is referred to as a UL grant), a set oftransmit power control commands for individual UEs in any UE group,and/or activation of a voice over Internet protocol (VoIP).

The 3GPP LTE uses blind decoding for PDCCH detection. The blind decodingis a scheme in which a desired identifier is de-masked from a cyclicredundancy check (CRC) of a received PDCCH (referred to as a candidatePDCCH) to determine whether the PDCCH is its own control channel byperforming CRC error checking.

The BS determines a PDCCH format according to DCI to be transmitted tothe UE, attaches a CRC to the DCI, and masks a unique identifier(referred to as a radio network temporary identifier (RNTI)) to the CRCaccording to an owner or usage of the PDCCH.

FIG. 2 shows an example of a UL subframe in 3GPP LTE.

The UL subframe can be divided into a control region and a data regionin a frequency domain. The control region is a region to which aphysical uplink control channel (PUCCH) carrying UL control informationis assigned. The data region is a region to which a physical uplinkshared channel (PUSCH) carrying user data is assigned.

The PUCCH is allocated in an RB pair in a subframe. RBs belonging to theRB pair occupy different subcarriers in each of a 1st slot and a 2^(nd)slot. m is a location index indicating a logical frequency-domainlocation of the RB pair allocated to the PUCCH in the subframe. It showsthat RBs having the same value m occupy different subcarriers in the twoslots.

According to 3GPP TS 36.211 V8.7.0, the PUCCH supports multiple formats.A PUCCH having a different number of bits per subframe can be usedaccording to a modulation scheme which is dependent on the PUCCH format.

Table 2 below shows an example of a modulation scheme and the number ofbits per subframe according to the PUCCH format.

TABLE 2 Number of bits PUCCH format Modulation scheme per subframe 1 N/A N/A 1a BPSK 1 1b QPSK 2 2  QPSK 20 2a QPSK + BPSK 21 2b QPSK + QPSK22

The PUCCH format 1 is used for transmission of a scheduling request(SR). The PUCCH formats 1a/1b are used for transmission of an ACK/NACKsignal. The PUCCH format 2 is used for transmission of a CQI. The PUCCHformats 2a/2b are used for simultaneous transmission of the CQI and theACK/NACK signal. When only the ACK/NACK signal is transmitted in asubframe, the PUCCH formats 1a/1b are used. When the SR is transmittedalone, the PUCCH format 1 is used. When the SR and the ACK/NACK aresimultaneously transmitted, the PUCCH format 1 is used, and in thistransmission, the ACK/NACK signal is modulated by using a resourceallocated to the SR.

All PUCCH formats use a cyclic shift (CS) of a sequence in each OFDMsymbol. The cyclically shifted sequence is generated by cyclicallyshifting a base sequence by a specific CS amount. The specific CS amountis indicated by a CS index.

An example of a base sequence r_(u)(n) is defined by Equation 1 below.

r _(u)(n)=e ^(jb(n)π/4)  [Equation 1]

In Equation 1, u denotes a root index, and n denotes a component indexin the range of 0≦n≦N−1, where N is a length of the base sequence. b(n)is defined in the section 5.5 of 3GPP TS 36.211 V8.7.0.

A length of a sequence is equal to the number of elements included inthe sequence. u can be determined by a cell identifier (ID), a slotnumber in a radio frame, etc. When it is assumed that the base sequenceis mapped to one RB in a frequency domain, the length N of the basesequence is 12 since one RB includes 12 subcarriers. A different basesequence is defined according to a different root index.

The base sequence r(n) can be cyclically shifted by Equation 2 below togenerate a cyclically shifted sequence r(n, l_(cs)).

$\begin{matrix}{{{r\left( {n,I_{cs}} \right)} = {{r(n)} \cdot {\exp \left( \frac{j\; 2\; \pi \; I_{cs}n}{N} \right)}}},{0 \leq I_{cs} \leq {N - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, l_(cs) denotes a CS index indicating a CS amount(0≦l_(cs)≦N−1).

Hereinafter, the available CS of the base sequence denotes a CS indexthat can be derived from the base sequence according to a CS interval.For example, if the base sequence has a length of 12 and the CS intervalis 1, the total number of available CS indices of the base sequence is12. Alternatively, if the base sequence has a length of 12 and the CSinterval is 2, the total number of available CS indices of the basesequence is 6.

Now, transmission of an HARQ ACK/NACK signal in the PUCCH format 1b willbe described.

FIG. 3 shows a PUCCH format 1b in a normal CP in 3GPP LTE.

One slot includes 7 OFDM symbols. Three OFDM symbols are used asreference signal (RS) OFDM symbols for a reference signal. Four OFDMsymbols are used as data OFDM symbols for an ACK/NACK signal.

In the PUCCH format 1b, a modulation symbol d(0) is generated bymodulating a 2-bit ACK/NACK signal based on quadrature phase shiftkeying (QPSK).

A CS index l_(cs) may vary depending on a slot number n_(s) in a radioframe and/or a symbol index I in a slot.

In the normal CP, there are four data OFDM symbols for transmission ofan ACK/NACK signal in one slot. It is assumed that CS indices mapped tothe respective data OFDM symbols are denoted by l_(cs0), l_(cs1),l_(cs2), and l_(cs3).

The modulation symbol d(0) is spread to a cyclically shifted sequencer(n,l_(cs)). When a one-dimensionally spread sequence mapped to an(i+1)^(th) OFDM symbol in a subframe is denoted by m(i), it can beexpressed as follows.

{m(0),m(1),m(2),m(3)}={d(0)r(n,l _(cs0)),d(0)r(n,l _(cs1)),d(0)r(n,l_(cs2)),d(0)r(n,l _(cs3))}

In order to increase UE capacity, the one-dimensionally spread sequencecan be spread by using an orthogonal sequence. An orthogonal sequencew_(i)(k) (where i is a sequence index, 0≦k≦K−1) having a spreadingfactor K=4 uses the following sequence.

TABLE 3 Index (i) [w_(i)(0), w_(i)(1), w_(i)(2), w_(i)(3)] 0 [+1, +1,+1, +1] 1 [+1, −1, +1, −1] 2 [+1, −1, −1, +1]

An orthogonal sequence w_(i)(k) (where i is a sequence index, 0≦k≦K−1)having a spreading factor K=3 uses the following sequence.

TABLE 4 Index (i) [w_(i)(0), w_(i)(1), w_(i)(2)] 0 [+1, +1, +1] 1 [+1,e^(j2π/3), e^(j4π/3)] 2 [+1, e^(j4π/3), e^(j2π/3)]

A different spreading factor can be used for each slot.

Therefore, when any orthogonal sequence index i is given, atwo-dimensionally spread sequences {s(0), s(1), s(2), s(3)} can beexpressed as follows.

{s(0),s(1),s(2),s(3)}={w _(i)(0)m(0),w _(i)(1)m(1),w _(i)(2)m(2),w_(i)(3)m(3)}

The two-dimensionally spread sequences {s(0), s(1), s(2), s(3)} aresubjected to inverse fast Fourier transform (IFFT) and thereafter aretransmitted in corresponding OFDM symbols. Accordingly, an ACK/NACKsignal is transmitted on a PUCCH.

A reference signal for the PUCCH format 1b is also transmitted bycyclically shifting the base sequence r(n) and then by spreading it bythe use of an orthogonal sequence. When CS indices mapped to three RSOFDM symbols are denoted by l_(cs4), l_(cs5), and l_(cs6), threecyclically shifted sequences r(n,l_(cs4)), r(n,l_(cs5)), andr(n,l_(cs6)) can be obtained. The three cyclically shifted sequences arespread by the use of an orthogonal sequence w^(RS) _(i)(k) having aspreading factor K=3.

An orthogonal sequence index i, a CS index l_(cs), and a resource blockindex m are parameters required to configure the PUCCH, and are alsoresources used to identify the PUCCH (or UE). If the number of availablecyclic shifts is 12 and the number of available orthogonal sequenceindices is 3, PUCCHs for 36 UEs in total can be multiplexed with oneresource block.

In the 3GPP LTE, a resource index n⁽¹⁾ _(PUCCH) is defined in order forthe UE to obtain the three parameters for configuring the PUCCH. Theresource index n⁽¹⁾ _(PUCCH) is defined to n_(CCE)+N⁽¹⁾ _(PUCCH), wheren_(CCE) is an index of a first CCE used for transmission ofcorresponding DCI (i.e., DL resource allocation used to receive DL datamapped to an ACK/NACK signal), and N⁽¹⁾ _(PUCCH) is a parameter reportedby a BS to the UE by using a higher-layer message.

Time, frequency, and code resources used for transmission of theACK/NACK signal are referred to as ACK/NACK resources or PUCCHresources. As described above, an index of the ACK/NACK resourcerequired to transmit the ACK/NACK signal on the PUCCH (referred to as anACK/NACK resource index or a PUCCH index) can be expressed with at leastany one of an orthogonal sequence index i, a CS index l_(cs), a resourceblock index m, and an index for obtaining the three indices. TheACK/NACK resource may include at least one of an orthogonal sequence, acyclic shift, a resource block, and a combination thereof.

FIG. 4 shows an example of performing HARQ.

By monitoring a PDCCH, a UE receives a DL grant including a DL resourceallocation on a PDCCH 501 in an n^(th) DL subframe. The UE receives a DLtransport block through a PDSCH 502 indicated by the DL resourceallocation.

The UE transmits an ACK/NACK response for the DL transport block on aPUCCH 511 in an (n+4)^(th) UL subframe. The ACK/NACK response can beregarded as a reception acknowledgement for the DL transport block.

The ACK/NACK signal corresponds to an ACK signal when the DL transportblock is successfully decoded, and corresponds to a NACK signal when theDL transport block fails in decoding. Upon receiving the NACK signal, aBS may retransmit the DL transport block until the ACK signal isreceived or until the number of retransmission attempts reaches itsmaximum number.

In the 3GPP LTE, to configure a resource index of the PUCCH 511, the UEuses a resource allocation of the PDCCH 501. That is, a lowest CCE index(or an index of a first CCE) used for transmission of the PDCCH 501 isn_(CCE), and the resource index is determined as n⁽¹⁾_(PUCCH)=n_(CCE)+N⁽¹⁾ _(PUCCH).

Now, ACK/NACK transmission for HARQ in 3GPP LTE time division duplex(TDD) will be described.

A UL subframe and a DL subframe coexist in one radio frame in the TDD,unlike in frequency division duplex (FDD). In general, the number of ULsubframes is less than the number of DL subframes. Therefore, inpreparation for a case in which the UL subframes for transmitting anACK/NACK signal are insufficient, it is supported that a plurality ofACK/NACK signals for a plurality of DL transport blocks are transmittedin one UL subframe.

According to the section 10.1 of 3GPP TS 36.213 V8.7.0 (2009-05), twoACK/NACK modes, i.e., channel selection and bundling, are introduced.

First, the bundling is an operation in which, if all of PDSCHs (i.e., DLtransport blocks) received by a UE are successfully decoded, ACK istransmitted, and otherwise NACK is transmitted. This is called an ANDoperation.

However, the bundling is not limited to the AND operation, and mayinclude various operations for compressing ACK/NACK bits correspondingto a plurality of transport blocks (or codewords). For example, thebundling may indicate a counter value indicating the number of ACKs (orNACKs) or the number of consecutive ACKs.

Second, the channel selection is also called ACK/NACK multiplexing. TheUE transmits the ACK/NACK by selecting one of a plurality of PUCCHresources.

Table 5 below shows a DL subframe n-k associated with a UL subframe ndepending on the UL-DL configuration in 3GPP LTE. Herein, kεK, where Mis the number of elements of a set K.

TABLE 5 UL-DL Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 — — 6 — 4— — 6 — 4 1 — — 7, 6 4 — — — 7, 6 4 — 2 — — 8, 7, 4, 6 — — — — 8, 7, — —4, 6 3 — — 7, 6, 11 6, 5 5, 4 — — — — — 4 — — 12, 8, 7, 11 6, 5, — — — —— — 4, 7 5 — — 13, 12, 9, 8, — — — — — — — 7, 5, 4, 11, 6 6 — — 7 7 5 —— 7 7 —

Assume that M DL subframes are associated with a UL subframe n, whereM=3. Since 3 PDCCHs can be received from 3 DL subframes, the UE canacquire 3 PUCCH resources n⁽¹⁾ _(PUCCH,0), n⁽¹⁾ _(PUCCH,1), n⁽¹⁾_(PUCCH,2). An example of channel selection is shown in Table 6 below.

TABLE 6 HARQ-ACK(0), HARQ-ACK(1), HARQ-ACK(2) n⁽¹⁾ _(PUCCH) b(0), b(1)ACK, ACK, ACK n⁽¹⁾ _(PUCCH, 2) 1, 1 ACK, ACK, NACK/DTX n⁽¹⁾ _(PUCCH, 1)1, 1 ACK, NACK/DTX, ACK n⁽¹⁾ _(PUCCH, 0) 1, 1 ACK, NACK/DTX, NACK/DTXn⁽¹⁾ _(PUCCH, 0) 0, 1 NACK/DTX, ACK, ACK n⁽¹⁾ _(PUCCH, 2) 1, 0 NACK/DTX,ACK, NACK/DTX n⁽¹⁾ _(PUCCH, 1) 0, 0 NACK/DTX, NACK/DTX, ACK n⁽¹⁾_(PUCCH, 2) 0, 0 DTX, DTX, NACK n⁽¹⁾ _(PUCCH, 2) 0, 1 DTX, NACK,NACK/DTX n⁽¹⁾ _(PUCCH, 1) 1, 0 NACK, NACK/DTX, NACK/DTX n⁽¹⁾ _(PUCCH, 0)1, 0 DTX, DTX, DTX N/A N/A

HARQ-ACK(i) denotes ACK/NACK for an i^(th) DL subframe among the M DLsubframes. Discontinuous transmission (DTX) implies that a DL transportblock cannot be received on a PDSCH in a corresponding DL subframe or acorresponding PDCCH cannot be detected. In Table 6 above, there arethree PUCCH resources n⁽¹⁾ _(PUCCH,0), n⁽¹⁾ _(PUCCH,1), and n⁽¹⁾_(PUCCH,2), and b(0) and b(1) are 2 bits transmitted by using a selectedPUCCH.

For example, if the UE successfully receives three DL transport blocksin three DL subframes, the UE transmits bits (1,1) through the PUCCH byusing n⁽¹⁾ _(PUCCH,2). If the UE fails to decode the DL transport blockand successfully decodes the remaining transport blocks in a 1^(st)(i=0) DL subframe, the UE transmits bits (0, 1) through the PUCCH byusing n⁽¹⁾ _(PUCCH,2).

In channel selection, NACK and DTX are coupled if at least one ACKexists. This is because a combination of a reserved PUCCH resource and aQPSK symbol is not enough to express all ACK/NACK states. However, ifthe ACK does not exist, the DTX and the NACK are decoupled.

The conventional PUCCH format 1b can transmit only 2-bit ACK/NACK.However, channel selection is used to express more ACK/NACK states bylinking the allocated PUCCH resources and an actual ACK/NACK signal.

Meanwhile, if it is assumed that M DL subframes are associated with a ULsubframe n, ACK/NACK may be mismatched between the BS and the UE due tomissing of the DL subframe (or PDCCH).

Assume that M=3, and the BS transmits three DL transport blocks throughthree DL subframes. The UE misses the PDCCH in the 2^(nd) DL subframeand thus cannot receive a 2^(nd) transport block at all, and can receiveonly the remaining 1st and 3^(rd) transport blocks. In this case, ifbundling is used, the UE erroneously transmits ACK.

In order to solve this error, a downlink assignment index (DAI) isincluded in a DL grant on the PDCCH. The DAI indicates an accumulativecounter value of the PDCCH which is related to a transmission of aPDSCH. A value of the 2-bit DAI is sequentially increased from 1, and amodulo-4 operation is applicable again from DAI=4. If M=5 and all of 5DL subframes are scheduled, the DAI can be included in a correspondingPDCCH in the order of DAI=1, 2, 3, 4, 1.

Now, a multiple-carrier system will be described.

A 3GPP LTE system supports a case in which a DL bandwidth and a ULbandwidth are differently configured under the premise that onecomponent carrier (CC) is used. The 3GPP LTE system supports up to 20MHz, and the UL bandwidth and the DL bandwidth may be different fromeach other. However, only one CC is supported in each of UL and DLcases.

Spectrum aggregation (also referred to as bandwidth aggregation orcarrier aggregation) supports a plurality of CCs. For example, if 5 CCsare assigned as a granularity of a carrier unit having a bandwidth of 20MHz, a bandwidth of up to 100 MHz can be supported.

One DL CC or a pair of a UL CC and a DL CC can be mapped to one cell.Therefore, when a UE communicates with a BS through a plurality of DLCCs, it can be said that the UE receives a service from a plurality ofserving cells.

FIG. 5 shows an example of multiple carriers.

Although three DL CCs and three UL CCs are shown herein, the number ofDL CCs and the number of UL CCs are not limited thereto. A PDCCH and aPDCCH are independently transmitted in each DL CC. A PUCCH and a PUSCHare independently transmitted in each UL CC. Since three DL CC-UL CCpairs are defined, it can be said that a UE receives a service fromthree serving cells.

The UE can monitor the PDCCH in a plurality of DL CCs, and can receive aDL transport block simultaneously via the plurality of DL CCs. The UEcan transmit a plurality of UL transport blocks simultaneously via aplurality of UL CCs.

It is assumed that a pair of a DL CC #1 and a UL CC #1 is a 1st servingcell, a pair of a DL CC #2 and a UL CC #2 is a 2^(nd) serving cell, anda DL CC #3 is a 3^(rd) serving cell. Each serving cell can be identifiedby using a cell index (CI). The CI may be cell-specific or UE-specific.Herein, CI=0, 1, 2 are assigned to the 1st to 3^(rd) serving cells forexample.

The serving cell can be classified into a primary cell and a secondarycell. The primary cell operates at a primary frequency, and is a celldesignated as the primary cell when the UE performs an initial networkentry process or starts a network re-entry process or performs ahandover process. The primary cell is also called a reference cell. Thesecondary cell operates at a secondary frequency. The secondary cell canbe configured after an RRC connection is established, and can be used toprovide an additional radio resource. At least one primary cell isconfigured always. The secondary cell can be added/modified/released byusing higher-layer signaling (e.g., RRC messages).

The CI of the primary cell may be fixed. For example, a lowest CI can bedesignated as a CI of the primary cell. It is assumed hereinafter thatthe CI of the primary cell is 0 and a CI of the secondary cell isallocated sequentially starting from 1.

The multiple carrier system can support non-cross carrier scheduling andcross carrier scheduling.

The non-cross carrier scheduling is a scheduling method in which a PDSCHand a PDCCH for scheduling the PDSCH are transmitted via the same DL CC.In addition, a DL CC in which a PDCCH for scheduling a PUSCH and a UL CCin which the PUSCH is transmitted are basically linked CCs in thisscheduling method.

The cross-carrier scheduling is a scheduling method capable ofperforming resource allocation of a PDSCH transmitted by using adifferent carrier through a PDCCH transmitted via a specific CC. Inaddition, the cross-carrier scheduling is a scheduling method capable ofperforming resource allocation of a PUSCH transmitted via another CCother than a CC basically linked to the specific CC. That is, the PDCCHand the PDSCH can be transmitted through different DL CCs, and the PUSCHcan be transmitted via a UL CC other than a UL CC linked to a DL CC onwhich a PDCCH including a UL grant is transmitted. In a systemsupporting the cross-carrier scheduling, a carrier indicator is requiredto report a specific DL CC/UL CC used to transmit the PDSCH/PUSCH forwhich the PDCCH provides control information. A field including thecarrier indicator is hereinafter called a carrier indication field(CIF).

In cross-carrier scheduling, a BS can determine a PDCCH monitoring DL CCset. The PDCCH monitoring DL CC set consists of some DL CCs among allaggregated DL CCs. When the cross-carrier scheduling is configured, a UEperforms PDCCH monitoring/decoding only for a DL CC included in thePDCCH monitoring DL CC set. The PDCCH monitoring DL CC set can bedetermined in a UE-specific, UE group-specific, or cell-specific manner.

FIG. 6 shows an example of cross-carrier scheduling in a multiplecarrier system.

Referring to FIG. 6, 3 DL CCs (i.e., DL CC A, DL CC B, DL CC C) areaggregated, and the DL CC A is determined as the PDCCH monitoring DL CC.The UE can receive a DL grant for a PDSCH of the DL CC A, the DL CC B,and the DL CC C through the PDCCH. A CIF may be included in DCItransmitted through the PDCCH of the DL CC A to indicate a specific DLCC for which the DCI is provided.

Now, semi-persistent scheduling (SPS) will be described.

In general, a UE first receives a DL grant on a PDCCH, and subsequentlyreceives a transport block through a PDSCH indicated by the DL grant.This implies that PDCCH monitoring is accompanied in every transportblock, which is called dynamic scheduling.

The SPS pre-defines a PDSCH resource, and the UE receives a transportblock through the pre-defined resource without PDCCH monitoring.

FIG. 7 shows an example of SPS in 3GPP LTE. Although DL SPS is shownherein, the same is also applicable to UL SPS.

First, a BS sends an SPS configuration to a UE by using radio resourcecontrol (RRC). The SPS configuration includes an SPS-C-RNTI and an SPSperiod. It is assumed herein that the SPS period is four subframes.

Even if the SPS is configured, the SPS is not immediately performed. TheUE monitors a PDCCH 501 in which a CRC is masked with the SPS-C-RNTI,and performs the SPS after the SPS is activated. When NDI=0 is includedin DCI on the PDCCH 501, combinations of values of several fields (e.g.,a transmit power command (TPC), a cyclic shift (CS) of a demodulationreference signal (DMSR), a modulation and coding scheme (MCS), aredundancy version (RV), an HARQ process number, and a resourceallocation) included in the DCI are used in SPS activation anddeactivation.

When the SPS is activated, even if a DL grant on the PDCCH is notreceived, the UE receives a transport block on a PDSCH at an SPS period.The PDSCH received without the PDCCH is called an SPS PDSCH. The PDCCHfor deactivating the SPS is called an SPS release PDCCH.

Thereafter, the UE monitors a PDCCH 502 in which a CRC is masked withthe SPS-C-RNTI, and confirms deactivation of the SPS.

According to 3GPP LTE, the PDCCH indicating the activation of the SPSdoes not require an ACK/NACK response, but the SPS release PDCCHindicating the deactivation of the SPS requires the ACK/NACK response.Hereinafter, a DL transport block may include the SPS release PDCCH.

According to the conventional PUCCH formats 1a/1b, a resource index n⁽¹⁾_(PUCCH) is acquired from the PDCCH. However, according to the SPS, thePDCCH associated with the PDSCH is not received, and thus a pre-assignedresource index is used.

Now, ACK/NACK transmission in a TDD system according to the presentinvention will be described.

An ACK/NACK state for HARQ indicates one of the following three states.

-   -   ACK: a decoding success of a transport block received on a        PDSCH.    -   NACK: a decoding failure of the transport block received on the        PDSCH.    -   DTX: a failure in the reception of the transport block on the        PDSCH. In case of dynamic scheduling, a failure in the reception        of a PDCCH.

As shown in Table 5, the M DL subframes are associated with the ULsubframe n according to the UL-DL configuration. In addition, in themultiple carrier system, M DL subframes in each of a plurality of DL CCscan be associated with a UL subframe n of one UL CC. In this case, thenumber of bits that can be transmitted in a UL subframe n in whichACK/NACK is transmitted may be less than the number of bits forexpressing all ACK/NACK states for a plurality of DL subframes.Therefore, in order to express the ACK/NACK by using a smaller number ofbits, an ACK/NACK multiplexing method can be considered as follows.

(1) Bundled ACK counter: A UE can deliver an ACK counter value to a BSonly when data received in each DL CC is transmitted without DTX and isall confirmed as ACK. That is, the UE delivers the ACK counter value as‘0’ when even one piece of received data is confirmed as NACK or DTX. Byusing a received DAI value, the UE can know a counter value of a PDSCH(excluding an SPS PDSCH) for which ACK/NACK is transmitted.

FIG. 8 shows an example of a method of using a bundled ACK counter.

Referring to FIG. 8, a DL CC #1 and a DL CC #2 are assigned to a UE. Inthe DL CC #1, if data is received in DL subframes #0, 2, and 3 and isall confirmed as ACK, the UE transmits information indicating that anACK counter value is 3. On the other hand, in the DL CC #2, data isreceived in DL subframes #0, 1, and 3 and data received in the DLsubframe #3 is confirmed as NACK. Therefore, the UE transmitsinformation indicating that the ACK counter value is 0.

(2) Consecutive ACK counter: The UE can deliver an accumulative ACKcounter value for subframes which are transmitted without DTX and whichare consecutively confirmed as ACK, starting from a first subframe in Msubframes of each DL CC.

FIG. 9 shows an example of a method of using a consecutive ACK counter.

Referring to FIG. 9, a DL CC #1 and a DL CC #2 are assigned to a UE. TheUE receives data in DL subframes #0, 2, and 3 of the DL CC #1, and threepieces of data are confirmed as data transmitted without DTX and isconsecutively confirmed as ACK. In this case, the UE transmits anaccumulative ACK counter value, i.e., 3, as a value indicating the ACKcounter value.

On the other hand, data is received in DL subframes #0, 1, and 2 of theDL CC #2, and data received in the DL subframes #0 and 1 is successfullydecoded and thus is confirmed as ACK, whereas data received in the DLsubframe #3 is confirmed as NACK. In this case, since two pieces of dataare consecutively confirmed as ACK, the accumulative ACK counter value,i.e., 2, is transmitted as an ACK counter value. Hereinafter, it isassumed in the present invention that a consecutive ACK counter is used.That is, a transmission method in which TDD HARQ-ACK is multiplexed byusing a TDD system, two serving cells, a consecutive ACK counter, and aPUCCH format 1b using channel selection is exemplified herein. However,the present invention is not limited thereto. That is, the presentinvention can be generally applied when channel selection is used in aTDD system which aggregates two serving cells.

In order to effectively deliver per-DL CC ACK counter value information,a channel selection method can be used. For the channel selection, aper-DL CC ACK counter value can be mapped to a state of Table 7 below.The state includes 2-bit information.

TABLE 7 State ACK counter value (B0, B1) or (B2, B3) 0 N, N or D, D 1 A,N 2 N, A 3 A, A 4 A, N 5 N, A 6 A, A 7 A, N 8 N, A 9 A, A

For example, assume that a DL CC #1 and a DL CC #2 are assigned to a UE,and M DL subframes linked to one UL subframe are 3 in number (i.e.,M=3). In this case, if three consecutive ACKs are generated in the DL CC#1 and two consecutive ACKs are generated in the DL CC #2, the UE mapsan ACK counter value (B0, B1) for the DL CC #1 to a state {A, A}, and anACK counter value (B1, B2) for the DL CC #2 is mapped to a state {N, A}.

Tables 8 and 9 below show a channel selection scheme used to deliver ACKcounter value information.

TABLE 8 B0 B1 B2 B3 Channel constellation D N/D N/D N/D NO TRANS- NOTRANS- MISSION MISSION N N/D N/D N/D H0  1 A N/D N/D N/D H0 −1 N/D A N/DN/D H1 −j A A N/D N/D H1  j N/D N/D A N/D H2  1 A N/D A N/D H2  j N/D AA N/D H2 −j A A A N/D H2 −1 N/D N/D N/D A H3  1 A N/D N/D A H0 −j N/D AN/D A H3  j A A N/D A H0  j N/D N/D A A H3 −j A N/D A A H3 −1 N/D A A AH1  1 A A A A H1 −1

TABLE 9 B0 B1 B2 B3 Channel constellation D D N/D N/D NO TRANS- NOTRANS- MISSION MISSION N N N/D N/D H0  1 N D N/D N/D H0  1 D N N/D N/DH0  1 A N/D N/D N/D H0 +j N/D A N/D N/D H0 −j A A N/D N/D H0 −1 N/D N/DA N/D H3 +j A N/D A N/D H2  1 N/D A A N/D H1  1 A A A N/D H1 +j N/D N/DN/D A H3  1 A N/D N/D A H2 +j N/D A N/D A H3 −j A A N/D A H2 −1 N/D N/DA A H3 −1 A N/D A A H2 −j N/D A A A H1 −j A A A A H1 −1

In Tables 8 and 9, H0, H1, H2, and H3 denote a PUCCH resource n⁽¹⁾_(PUCCH) for channel selection. That is, H0 denotes n⁽¹⁾ _(PUCCH,0), H1denotes n⁽¹⁾ _(PUCCH,1), H2 denotes n⁽¹⁾ _(PUCCH,2), and H3 denotes n⁽¹⁾_(PUCCH,3) (the same is applied hereinafter). In addition, in signalconstellation, 1 indicates ‘00’, −1 indicates ‘11’, j indicates ‘10’,and −j indicates ‘01’.

When H0 to H3 and signal constellation are expressed as described above,Table 8 above can be expressed as shown in Table 10 and Table 11 below.Table 10 shows a case of M=3, and Table 11 shows a case of M=4.

TABLE 10 primary cell secondary cell signal HARQ-ACK(0), HARQ-ACK(0),constel- HARQ-ACK(1), HARQ-ACK(1), resource lation HARQ-ACK(2)HARQ-ACK(2) n⁽¹⁾ _(PUCCH) b(0), b(1) A, A, A A, A, A n⁽¹⁾ _(PUCCH, 1) 1,1 A, A, N/D A, A, A n⁽¹⁾ _(PUCCH, 1) 0, 0 A, N/D, any A, A, A n⁽¹⁾_(PUCCH, 3) 1, 1 N/D, any, any A, A, A n⁽¹⁾ _(PUCCH, 3) 0, 1 A, A, A A,A, N/D n⁽¹⁾ _(PUCCH, 0) 1, 0 A, A, N/D A, A, N/D n⁽¹⁾ _(PUCCH, 3) 1, 0A, N/D, any A, A, N/D n⁽¹⁾ _(PUCCH, 0) 0, 1 N/D, any, any A, A, N/D n⁽¹⁾_(PUCCH, 3) 0, 0 A, A, A A, N/D, any n⁽¹⁾ _(PUCCH, 2) 1, 1 A, A, N/D A,N/D, any n⁽¹⁾ _(PUCCH, 2) 0, 1 A, N/D, any A, N/D, any n⁽¹⁾ _(PUCCH, 2)1, 0 N/D, any, any A, N/D, any n⁽¹⁾ _(PUCCH, 2) 0, 0 A, A, A N/D, any,any n⁽¹⁾ _(PUCCH, 1) 1, 0 A, A, N/D N/D, any, any n⁽¹⁾ _(PUCCH, 1) 0, 1A, N/D, any N/D, any, any n⁽¹⁾ _(PUCCH, 0) 1, 1 N/D, any, any N/D, any,any n⁽¹⁾ _(PUCCH, 0) 0, 0 DTX, any, any N/D, any, any no transmission

TABLE 11 Primary Cell Secondary Cell HARQ-ACK(0), HARQ-ACK(0),HARQ-ACK(1), HARQ-ACK(1), Constel- HARQ-ACK(2), HARQ-ACK(2), Resourcelation HARQ-ACK(3) HARQ-ACK(3) n_(PUCCH) ⁽¹⁾ b(0), b(1) ACK, ACK, ACK,ACK, ACK, ACK, n_(PUCCH, 1) ⁽¹⁾ 1, 1 NACK/DTX NACK/DTX ACK, ACK, ACK,ACK, ACK, n_(PUCCH, 1) ⁽¹⁾ 0, 0 NACK/DTX, any NACK/DTX ACK, DTX, DTX,ACK, ACK, ACK, n_(PUCCH, 3) ⁽¹⁾ 1, 1 DTX NACK/DTX ACK, ACK, ACK, ACK,ACK, ACK, n_(PUCCH, 3) ⁽¹⁾ 1, 1 ACK NACK/DTX NACK/DTX, any, ACK, ACK,ACK, n_(PUCCH, 3) ⁽¹⁾ 0, 1 any, any NACK/DTX (ACK, NACK/DTX, ACK, ACK,ACK, n_(PUCCH, 3) ⁽¹⁾ 0, 1 any, any), NACK/DTX except for (ACK, DTX,DTX, DTX) ACK, ACK, ACK, ACK, ACK, n_(PUCCH, 0) ⁽¹⁾ 1, 0 NACK/DTXNACK/DTX, any ACK, ACK, ACK, ACK, n_(PUCCH, 3) ⁽¹⁾ 1, 0 NACK/DTX, anyNACK/DTX, any ACK, DTX, DTX, ACK, ACK, n_(PUCCH, 0) ⁽¹⁾ 0, 1 DTXNACK/DTX, any ACK, ACK, ACK, ACK, ACK, n_(PUCCH, 0) ⁽¹⁾ 0, 1 ACKNACK/DTX, any NACK/DTX, any, ACK, ACK, n_(PUCCH, 3) ⁽¹⁾ 0, 0 any, anyNACK/DTX, any (ACK, NACK/DTX, ACK, ACK, n_(PUCCH, 3) ⁽¹⁾ 0, 0 any, any),NACK/DTX, any except for (ACK, DTX, DTX, DTX) ACK, ACK, ACK, ACK, DTX,DTX, n_(PUCCH, 2) ⁽¹⁾ 1, 1 NACK/DTX DTX ACK, ACK, ACK, ACK, ACK, ACK,n_(PUCCH, 2) ⁽¹⁾ 1, 1 NACK/DTX ACK ACK, ACK, ACK, DTX, DTX, n_(PUCCH, 2)⁽¹⁾ 0, 1 NACK/DTX, any DTX ACK, ACK, ACK, ACK, ACK, n_(PUCCH, 2) ⁽¹⁾ 0,1 NACK/DTX, any ACK ACK, DTX, DTX, ACK, DTX, DTX, n_(PUCCH, 2) ⁽¹⁾ 1, 0DTX DTX ACK, DTX, DTX, ACK, ACK, ACK, n_(PUCCH, 2) ⁽¹⁾ 1, 0 DTX ACK ACK,ACK, ACK, ACK, DTX, DTX, n_(PUCCH, 2) ⁽¹⁾ 1, 0 ACK DTX ACK, ACK, ACK,ACK, ACK, ACK, n_(PUCCH, 2) ⁽¹⁾ 1, 0 ACK ACK NACK/DTX, any, ACK, DTX,DTX, n_(PUCCH, 2) ⁽¹⁾ 0, 0 any, any DTX NACK/DTX, any, ACK, ACK, ACK,n_(PUCCH, 2) ⁽¹⁾ 0, 0 any, any ACK (ACK, NACK/DTX, ACK, DTX, DTX,n_(PUCCH, 2) ⁽¹⁾ 0, 0 any, any), DTX except for (ACK, DTX, DTX, DTX)(ACK, NACK/DTX, ACK, ACK, ACK, n_(PUCCH, 2) ⁽¹⁾ 0, 0 any, any), ACKexcept for (ACK, DTX, DTX, DTX) ACK, ACK, ACK, NACK/DTX, any,n_(PUCCH, 1) ⁽¹⁾ 1, 0 NACK/DTX any, any ACK, ACK, ACK, (ACK, NACK/DTX,n_(PUCCH, 1) ⁽¹⁾ 1, 0 NACK/DTX any, any), except for (ACK, DTX, DTX,DTX) ACK, ACK, NACK/DTX, any, n_(PUCCH, 1) ⁽¹⁾ 0, 1 NACK/DTX, any any,any ACK, ACK, (ACK, NACK/DTX, n_(PUCCH, 1) ⁽¹⁾ 0, 1 NACK/DTX, any any,any), except for (ACK, DTX, DTX, DTX) ACK, DTX, DTX, NACK/DTX, any,n_(PUCCH, 0) ⁽¹⁾ 1, 1 DTX any, any ACK, DTX, DTX, (ACK, NACK/DTX,n_(PUCCH, 0) ⁽¹⁾ 1, 1 DTX any, any), except for (ACK, DTX, DTX, DTX)ACK, ACK, ACK, NACK/DTX, any, n_(PUCCH, 0) ⁽¹⁾ 1, 1 ACK any, any ACK,ACK, ACK, (ACK, NACK/DTX, n_(PUCCH, 0) ⁽¹⁾ 1, 1 ACK any, any), exceptfor (ACK, DTX, DTX, DTX) NACK, any, any, NACK/DTX, any, n_(PUCCH, 0) ⁽¹⁾0, 0 any any, any NACK, any, any, (ACK, NACK/DTX, n_(PUCCH, 0) ⁽¹⁾ 0, 0any any, any), except for (ACK, DTX, DTX, DTX) (ACK, NACK/DTX, NACK/DTX,any, n_(PUCCH, 0) ⁽¹⁾ 0, 0 any, any), any, any except for (ACK, DTX,DTX, DTX) (ACK, NACK/DTX, (ACK, NACK/DTX, n_(PUCCH, 0) ⁽¹⁾ 0, 0 any,any), any, any), except for (ACK, except for (ACK, DTX, DTX, DTX) DTX,DTX, DTX) DTX, any, any, NACK/DTX, any, No Transmission any any, anyDTX, any, any, (ACK, NACK/DTX, No Transmission any any, any), except for(ACK, DTX, DTX, DTX)

In Tables 10 and 11 above, first and second cells respectively indicateprimary and secondary cells. HARQ-ACK(j) denotes ACK/NACK correspondingto a PDSCH scheduled by a PDCCH of which a DAI value is j+1, or denotesACK/NACK corresponding to a PDCCH which requests an ACK/NACK response,for example, an SPS release PDCCH indicating a release ofsemi-persistent scheduling (herein, j is 0≦j≦M−1). However, if the SPSPDSCH exists, HARQ-ACK(0) denotes ACK/NACK for an SPS PDSCH, andHARQ-ACK(j>0) denotes ACK/NACK corresponding to a PDSCH scheduled by aPDCCH of which a DAI value is j.

Hereinafter, it is described a method of allocating resources totransmit ACK/NACK by multiplexing the ACK/NACK with a PUCCH format 1 busing the aforementioned channel selection. It is assumed hereinafterthat a TDD mode is used, M is greater than 2, and two serving cells areconfigured. As described above, M is the number of DL subframescorresponding to one UL subframe in each DL CC. In this case, for thechannel selection, ACK/NACK information is transmitted by selecting anyone of 4 resources n⁽¹⁾ _(PUCCH,0), n⁽¹⁾ _(PUCCH,1), n⁽¹⁾ _(PUCCH,2),and n⁽¹⁾ _(PUCCH,3). In this case, which method will be used to allocatethe two resources is a matter to be considered.

[Resource Allocation Method in Channel Selection when ConfiguringCross-Carrier Scheduling]

1. When there is no SPS PDSCH transmission.

When cross-carrier scheduling is configured, a UE receives a PDCCH forscheduling a PDSCH and an SPS release PDCCH only in a primary cell. Ifthere is no SPS PDSCH transmission in the primary cell or if there is nosubframe configured to receive the SPS PDSCH, a resource used in channelselection can be allocated dynamically.

That is, two dynamic resources linked to two PDCCHs having the smallestDAI values among PDCCHs for scheduling the primary cell and two dynamicresources linked to two PDCCH having the smallest DAI values amongPDCCHs for scheduling a secondary cell can be allocated for channelselection. Herein, the PDCCH for scheduling the primary cell include notonly a normal PDCCH for scheduling a PDSCH but also any PDCCH (e.g., anSPS release PDCCH) for requiring an ACK/NACK response. Although thenormal PDCCH for scheduling the PDSCH and the SPS release PDCCH will beexemplified in the description of the present invention hereinafter, thepresent invention is not limited thereto, and thus any PDCCH forrequesting an ACK/NACK response can also be included.

For example, if the UE detects a PDCCH of which a DAI value is 1 or 2 ina subframe n-k_(m) of the primary cell and receives a PDSCH indicated bythe PDCCH in the primary cell, or if the UE detects an SPS release PDCCHof which a DAI value is 1 or 2 in the subframe n-k_(m) of the primarycell, a PUCCH resource n⁽¹⁾ _(PUCCH,1) for transmitting ACK/NACK can beallocated as shown in equation 3 below. Herein, k_(m)εK, and a DAI valueof a PDCCH at k_(m) is 1 or 2. K is described above with reference toTable 5.

n ⁽¹⁾ _(PUCCH,i)=(M−m−1)×N _(c) +m×N _(c+1) +n _(CCE,m) +N ⁽¹⁾_(PUCCH)  [Equation 3]

Herein, c is selected from {0,1,2,3} to satisfy N_(c)≦n_(CCE,m)<N_(c+1).N⁽¹⁾ _(PUCCH) is a value determined by using a higher layer signal.N_(C) may be max{0, floor [N^(DL) _(RB)×(N^(RB) _(sc)×c−4)/36]}. N^(DL)_(RB) is the number of RBs based on a configured DL bandwidth, andN^(RB) _(sc) is a size of a resource block indicated with the number ofsubcarriers in the frequency domain. n_(CCE,m) is a first CCE numberused in transmission of a corresponding PDCCH at a subframe n-k_(m).

In Equation 3, n⁽¹⁾ _(PUCCH,0), that is, i=0, denotes a PUCCH resourcedynamically determined in association with a PDCCH (i.e., a PDCCH forscheduling a primary cell) of which a DAI value is 1, and n⁽¹⁾_(PUCCH,1), that is, i=1, denotes a PUCCH resource dynamicallydetermined in association with a PDCCH (i.e., a PDCCH for scheduling aprimary cell) of which a DAI value is 2.

If the UE detects a PDCCH of which a DAI value is 1 or 2 in the subframen-k_(m) of the primary cell and receives a PDSCH indicated by the PDCCHin the secondary cell, a PUCCH resource is allocated according toEquation 3 above. In this case, the PDCCH is a PDCCH for scheduling aPDSCH transmitted in the secondary cell. That is, n⁽¹⁾ _(PUCCH,2), thatis, i=2, denotes a PUCCH resource dynamically determined in associationwith a PDCCH (i.e., a PDCCH for scheduling a secondary cell) of which aDAI value is 1, and n⁽¹⁾ _(PUCCH,3), that is, i=3, denotes a PUCCHresource dynamically determined in association with a PDCCH (i.e., aPDCCH for scheduling a secondary cell) of which a DAI value is 2.

FIG. 10 shows an ACK/NACK resource allocation method in case of theaforementioned cross carrier scheduling.

Referring to FIG. 10, since a PDCCH of which a DAI is 1 is received in aDL subframe #0 of a primary cell, H0 (i.e., n⁽¹⁾ _(PUCCH,0)) linked tothe PDCCH is allocated. In addition, since a PDCCH of which a DAI is 2is received in a DL subframe #2, H1 (i.e., n⁽¹⁾ _(PUCCH,1)) linked tothe PDCCH is allocated. In addition, if DAI values of PDCCHs forscheduling a PDSCH of DL subframes #0 and #1 of the secondary cellcorrespond to 1 and 2 in that order, H2 (i.e., n⁽¹⁾ _(PUCCH,2)) and H3(i.e., n⁽¹⁾ _(PUCCH,3)) linked to the corresponding PDCCHs areallocated.

FIG. 11 shows an example in which an ACK/NACK resource allocation methodis modified in case of the aforementioned cross carrier scheduling.

FIG. 11 differs from FIG. 10 in that a UE fails to receive a PDCCH withDAI=2 among PDCCHs for scheduling a primary cell. In this case, the UEallocates only a resource H0 linked to a PDCCH with DAI=1 and resourcesH2 and H3 linked to PDCCHs with DAI=1 and DAI=2 among PDCCHs forscheduling a secondary cell. There is no problem even if the UE does notallocate a resource H1 linked to the PDCCH with DAI=2 (for schedulingthe primary cell). This is because, as shown in Table 8 above, theresource 1(H1) is used when ACK is transmitted for a PDSCH schedule bythe PDCCH with DAI=2. However, the UE fails to receive the PDCCH withDAI=2, and thus there is no case where ACK is transmitted for the PDSCHscheduled by the PDCCH with DAI=2. Eventually, it is not a problem evenif PUCCH resource allocation recognition is mismatched between the BSand the UE.

2. When there is SPS PDSCH transmission

If an SPS PDSCH is included in DL subframes of a primary cell, aresource for channel selection can be allocated as follows.

The SPS PDSCH does not have a PDCCH for scheduling. Thus, a resource forchannel selection is reserved through a higher layer signal, and thereserved resource can be allocated to H0 (i.e., n⁽¹⁾ _(PUCCH,0)). Forexample, four resources (i.e., a first PUCCH resource, a second PUCCHresource, a third PUCCH resource, and a fourth PUCCH resource) can bereserved by using an RRC signal, and one resource can be indicated byusing a transmission power control (TPC) field of a PDCCH for activatingSPS scheduling.

Table 12 below shows an example of indicating a resource for channelselection according to the TPC field value.

TABLE 12 TPC field value Resource for channel selection ‘00’ 1^(st)PUCCH resource ‘01’ 2^(nd) PUCCH resource ‘10’ 3^(rd) PUCCH resource‘11’ 4^(th) PUCCH resource

A resource linked to a PDCCH (including an SPS release PDCCH) of which aDAI is 1 in a primary cell is allocated to H1 (i.e., n⁽¹⁾ _(PUCCH,1)).Dynamic resources linked to PDCCHs with DAI=1 and DAI=2 among PDCCHs forscheduling a secondary cell are respectively H2 (i.e., n⁽¹⁾ _(PUCCH,2))and H3 (i.e., n⁽¹⁾ _(PUCCH,3)). In this case, Equation 3 above can beused.

FIG. 12 shows an example of an ACK/NACK resource allocation method whenthere is SPS PDSCH transmission in case of cross carrier scheduling. Itis assumed in FIG. 12 that channel selection is performed according toTable 8 above.

Referring to FIG. 12, a UE allocates a reserved resource to H0 by usinga higher layer signal when an SPS PDSCH is received in a DL subframe #3of a primary cell. A resource linked to a PDCCH with DAI=1 in theprimary cell is allocated to H1. A resource linked to a PDCCH with DAI=1among PDCCHs for scheduling a secondary cell is allocated to H2. Aresource linked to a PDCCH with DAI=2 is allocated to H3.

In case of using channel selection based on Table 9, the resource H3 canbe modified in such a manner that dynamic signaling of a PDCCH isselected after securing the resource in advance by using a higher layersignal.

If the UE fails to receive PDCCHs with DAI=1 and DAI=2, sincecorresponding resources are not used in mapping according to thecharacteristics described in Table 8 to Table 10, the correspondingresources may be left unused while using only the remaining resources inthe channel selection.

For ACK/NACK detection, a BS can detect ACK/NACK in a channel selectionmanner by searching for only PUCCH format 1a/1b resources allocated withSPS and resources linked to PDCCHs with DAI=1 and 2 among PDCCHstransmitted from the BS. According to this method, mismatch of PUCCHresources can be avoided.

[Resource Allocation Method in Channel Selection when Non-Cross CarrierScheduling is Configured]

When non-cross carrier scheduling is configured, a PDCCH (or an SPSrelease PDCCH) for scheduling a PDSCH transmitted in a primary cell istransmitted in the primary cell, and a PDCCH for scheduling a PDSCHtransmitted in a secondary cell is transmitted in the secondary cell. Inthis case, four resources for channel selection are allocated by usingthe following method.

First, if there is no SPS PDSCH transmission in the primary cell, tworesources linked to PDCCHs with DAI value 1 and 2 among PDCCHs(including an SPS release PDCCH) for scheduling a PDSCH transmitted inthe primary call are allocated to H0 and H1. In this case, Equation 3can be used.

If an SPS PDSCH is included in DL subframes of the primary cell, aresource for channel selection can be reserved through a higher layersignal, and the reserved resource can be allocated to H0 (i.e., n⁽¹⁾_(PUCCH,0)). For example, four resources (i.e., a first PUCCH resource,a second PUCCH resource, a third PUCCH resource, and a fourth PUCCHresource) can be reserved by using an RRC signal, and one resource canbe indicated by using a transmission power control (TPC) field of aPDCCH for activating SPS scheduling. In addition, a resource linked to aPDCCH (including an SPS release PDCCH) of which a DAI is 1 in a primarycell is allocated to H1 (i.e., n⁽¹⁾ _(PUCCH,1)).

Regarding the remaining two resources H2 and H3, a plurality ofresources are reserved by using a higher layer signal and thereafter tworesources are selected from the plurality of resources. In this case,the two resources can be selected from the plurality of resources bydedicatedly using a TPC field included in a PDCCH for scheduling thesecondary cell as an ACK/NACK resource indicator (ARI).

For example, an RRC signal can be used to reserve four resource pairs(i.e., 8 resources in total) and thereafter any one resource pair can beindicated among the four resource pairs according to a bit value of a2-bit TPC field.

In this case, all PDCCHs for scheduling the secondary cell may have thesame value in the TPC field in M corresponding DL subframes of thesecondary cell, and the UE can assume that the all PDCCHs have the samevalue in the TPC field.

Alternatively, among the PDCCHs for scheduling the secondary cell, onlya TPC field of a PDCCH with DAM can be used dedicatedly for an ARI, anda TPC field of a PDCCH of which a DAI value is greater than 1 can beused for its original usage, i.e., for transmit power control. If the UEfails to receive the PDCCH with DAI=1, the UE transmits ‘0’ as an ACKcounter value. Referring to Table 8 above, ACK/NACK of which an ACKcounter value of a secondary cell is 0 uses only H0 and H1, and thusallocation of resources such as H2 and H3 are not necessary.

For another example, the RRC signal can be used to reserve 8 resourcesand thereafter two resources can be indicated by using two 2-bit TPCfields. Among PDCCHs for scheduling a secondary cell, a TPC field of aPDCCH with DAI=1 and a TPC field of a PDCCH with DAI=2 can be used.Among the PDCCHs for scheduling the secondary cell, a TPC field is usedfor its original usage with respect to a PDCCH of which a DAI value isgreater than or equal to 2. According to this method, since each TPCfield indicates one of the four resources, two resources H2 and H3 canbe indicated independently by using two TPC fields. Therefore, resourceutilization of the BS can be increased.

Hereinafter, a case in which ACK/NACK for two DL subframes istransmitted in one UL subframe in each DL CC of a multiple carriersystem, that is, a case of M=2, will be described.

For example, it is assumed that a UE aggregates two DL CCs and a DLsubframe (SF):UL SF=2:1 is satisfied (i.e., two DL SFs are linked to oneUL SF). If both of the two DL CCs are not set to a MIMO mode, 4-bitACK/NCK can be transmitted by using 4-bit channel selection withouthaving to perform bundling.

If any one of two DL CCs is set to the MIMO mode, 4-bit ACK/NACK whichis bundled using spatial bundling can be transmitted through channelselection. Herein, the spatial bundling implies that an AND operation isperformed on ACK/NACK for a plurality of transmission blocks (orcodewords) received in the same subframe.

The UE can aggregate two DL CCs, and in case of DL SF:UL SF=2:1, cantransmit ACK/NACK by using channel selection. A resource allocationmethod for channel selection in this case will be described. Thisresource allocation method is a method of preventing a problem occurringin ACK/NACK transmission when the number of DL CCs recognized by the BSdiffers from the number of DL CCs recognized by the UE or when a ratioof DL SF:UL SF is recognized differently between the BS and the UE.

Mapping used for transmission of 2-bit ACK/NACK is as shown in Table 13below.

TABLE 13 B0 B1 Channel constellation D N/D NO TRANS- NO TRANS- MISSIONMISSION N N/D H0  1 A N/D H0 −1 N/D A H1 −j A A H1  j

Method A. When cross-carrier scheduling is configured.

FIG. 13 shows an example of resource allocation for channel selectionwhen cross-carrier scheduling is configured.

When the cross-carrier scheduling is configured, PDCCHs for scheduling aprimary cell and PDCCHs for scheduling a secondary cell are alltransmitted through the primary cell. Among the PDCCHs for schedulingthe primary cell, a resource linked to a first PDCCH (e.g., included ina DL SF #0) is allocated to H0, a resource linked to a second PDCCH(e.g., included in a DL SF #1) is allocated to H1. Among the PDCCHs forscheduling the secondary cell, a resource linked to a first PDCCH (e.g.,a DL SF #0) is allocated to H2, and a resource linked to a second PDCCH(e.g., a DL SF #1) is allocated to H3. If the UE fails to receive aPDCCH for scheduling a specific CC in a specific subframe, acorresponding resource is not used in channel selection, and thecorresponding resource may be left unused and channel selection isperformed by using only the remaining secured resources.

By allocating resources in this manner, ACK/NACK can be transmitted inan error-free manner even if the number of assigned DL CCs is recognizeddifferently between the BS and the UE. That is, an error does not occureven if the UE performs channel selection by using Table 8 and the BSmisunderstands that the UE performs the channel selection by using Table13. This is because a resource, a signal constellation, etc., of Table13 are the same as those of a case where ACK/NACK of the secondary cellis all N/D (i.e., a state indicating that an ACK counter value of thesecondary cell is 0) in Table 8.

FIG. 14 shows another example of resource allocation for channelselection when cross carrier scheduling is configured.

Among PDCCHs for scheduling a primary cell, a resource linked to a firstPDCCH (e.g., included in a DL SF #0) is allocated to H0, and a resourcelinked to a second PDCCH (e.g., included in a DL SF #1) is allocated notto H1 but to H2. Among PDCCHs for scheduling a secondary cell, aresource linked to a first PDCCH (e.g., included in a DL SF #0) isallocated not to H2 but to H1, and a resource liked to a second PDCCH(e.g., included in a DL SF #1) is allocated to H3. If a UE fails toreceive a PDCCH for scheduling a specific CC in a specific subframe, acorresponding resource is not used in channel selection, and thecorresponding resource may be left unused and channel selection isperformed by using only the remaining secured resources. By allocatingresources in this manner, ACK/NACK can be transmitted in an error-freemanner even if a value M, i.e., the number of DL subframes mapped to oneUL subframe, determined between a BS and the UE is incorrectlyrecognized. For example, an error does not occur even if the UErecognizes a ratio of DL SF:UL SF as 2:1 and thus uses Table 8 as achannel selection table, whereas the BS recognizes the ratio of DL SF:ULSF as 1:1 and thus uses Table 13 as the channel selection table.

Method B. When non-cross carrier scheduling is configured.

FIG. 15 shows an example of a resource allocation method when non-crosscarrier scheduling is configured.

If a PDCCH for scheduling a primary cell exists in a DL subframe #0, adynamic resource linked to the PDCCH is allocated to H0. If a PDCCH forscheduling the primary cell exists in a DL subframe #1, a dynamicresource H1 linked to the PDCCH is allocated to H1.

In addition, a dynamic resource linked to a PDCCH for scheduling asecondary cell is not used in channel selection. Instead, resources forthe channel selection are selected in such a manner that resources forthe secondary cell are reserved in advance by using a higher layersignal and a TPC included in a PDCCH for scheduling the secondary cellis dedicatedly used as an ARI.

In this case, two resources can be selected by dedicatedly using a TPCfield included in the PDCCH for scheduling the secondary cell as an ARI.For example, an RRC signal can be used to reserve four resource pairs(i.e., 8 resources in total) and thereafter any one resource pair can beindicated among the four resource pairs according to a bit value of a2-bit TPC field. In this case, among the PDCCHs for scheduling thesecondary cell, only a TPC field of a PDCCH with DAI=1 can be useddedicatedly for an ARI, and a TPC field of a PDCCH of which a DAI valueis greater than 1 can be used for its original usage, i.e., for transmitpower control.

For another example, the RRC signal can be used to reserve 8 resourcesand thereafter two resources can be indicated by using two 2-bit TPCfields. Among PDCCHs for scheduling a secondary cell, a TPC field of aPDCCH with DAI=1 and a TPC field of a PDCCH with DAI=2 can be used.Among the PDCCHs for scheduling the secondary cell, a TPC field is usedfor its original usage with respect to a PDCCH of which a DAI value isgreater than or equal to 2. According to this method, since each TPCfield indicates one of the four resources, two resources H2 and H3 canbe indicated independently by using two TPC fields. Therefore, resourceutilization of the BS can be increased.

FIG. 16 is a block diagram of a wireless apparatus for implementing anembodiment of the present invention.

A UE 20 includes a memory 22, a processor 21, and a radio frequency (RF)unit 23. The memory 22 coupled to the processor 21 stores a variety ofinformation for driving the processor 21. The RF unit 23 coupled to theprocessor 21 transmits and/or receives a radio signal. The processor 21implements the proposed functions, procedure, and/or methods. In theaforementioned embodiments, an operation of the UE can be implemented bythe processor 21. The processor 21 receives M DL subframes associatedwith a UL subframe n in each of two serving cells, and determines fourcandidate resources on the basis of the M DL subframes received in eachof the two serving cells. Further, the processor 21 transmits anACK/NACK response for the M DL subframes received in each of the twoserving cells by using one resource selected from the four candidateresources in the UL subframe n. In this case, the two serving cellsconsist of a first serving cell and a second serving cell, and among thefour candidate resources, a first resource and a second resource arerelated to a physical downlink shared channel (PDSCH) received in thefirst serving cell or a semi-persistent scheduling (SPS) release PDCCHfor releasing semi-persistent scheduling, and a third resource and afourth resources are related to a PDSCH received in the second servingcell.

Further, the processor 21 configures ACK/NACK, and transmits theACK/NACK through a PUSCH or a PUCCH.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The RF unit may include a base-bandcircuit for processing a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory and may be performed by the processor. The memory may be locatedinside or outside the processor, and may be coupled to the processor byusing various well-known means.

Although the aforementioned exemplary system has been described on thebasis of a flowchart in which steps or blocks are listed in sequence,the steps of the present invention are not limited to a certain order.Therefore, a certain step may be performed in a different step or in adifferent order or concurrently with respect to that described above.Further, it will be understood by those ordinary skilled in the art thatthe steps of the flowcharts are not exclusive. Rather, another step maybe included therein or one or more steps may be deleted within the scopeof the present invention.

1. A method of transmitting positive-acknowledgement (ACK)/negative-acknowledgement (NACK) in a time division duplex (TDD)-based wireless communication system in which M (M>2) downlink subframes are associated with an uplink subframe in each of two serving cells, the method performed by a user equipment (UE) and comprising: receiving M downlink subframes associated with an uplink subframe n in each of the two serving cells; and transmitting an ACK/NACK response for the M downlink subframes received in each of the two serving cells by using one resource selected from four candidate resources in the uplink subframe n, wherein the two serving cells comprises a first serving cell and a second serving cell, wherein, in the M downlink subframes received in the first serving cell, if a physical downlink shared channel (PDSCH) indicated by detecting a first physical downlink control channel (PDCCH) with a downlink assignment index (DAI) value equal to 1 or a second PDCCH with a DAI value equal to 2 is received, or if a first semi-persistent scheduling (SPS) release PDCCH with a DAI value equal to 1 or a second SPS release PDCCH with a DAI value equal to 2 is received, among the four candidate resources, a first resource is determined based on a first control channel element (CCE) used in transmission of the first PDCCH or the first SPS release PDCCH, and a second resource is determined based on a first CCE used in transmission of the second PDCCH or the second SPS release PDCCH.
 2. The method of claim 1, wherein at least one downlink subframe among the M downlink subframes received in the first serving cell comprises a physical downlink control channel (PDCCH) in which a downlink grant is transmitted and a PDSCH corresponding to the PDCCH.
 3. The method of claim 2, wherein the downlink grant includes a downlink assignment index (DAI) indicating an accumulative number of PDCCH with assigned PDSCH transmission.
 4. The method of claim 1, wherein if a PDSCH, indicated by detection of a third PDCCH with a DAI value equal to 1 or a fourth PDCCH with a DAI value equal to 2 received in the M downlink subframes in the first serving cell, is received in the M downlink subframes in the second serving cell, a third resource among the four candidate resources is determined based on a first CCE used in transmission of the third PDCCH, and a fourth resource among the four candidate resources is determined based on a first CCE used in transmission of the fourth PDCCH.
 5. The method of claim 1, wherein if a semi-persistent scheduling (SPS) physical downlink shared channel (PDSCH), which is a PDSCH having no corresponding physical downlink control channel (PDCCH), is received in the M downlink subframes received in the first serving cell, a first resource among the four candidate resources is one resource selected from four resources configured by using a higher layer signal, and the selected one resource is indicated by a PDCCH indicating activation of semi-persistent scheduling.
 6. The method of claim 5, wherein in the M downlink subframes received in the first serving cell, if a PDSCH indicated by detecting a first PDCCH with a DAI value equal to 1 or if a first SPS release PDCCH with a DAI value equal to 1 is received, a second resource among the four candidate resources is determined based on a first CCE used in transmission of the first PDCCH or the first SPS release PDCCH.
 7. A user equipment (UE) operated in a time division duplex (TDD)-based wireless communication system in which M (M>2) downlink subframes are associated with an uplink subframe in each of two serving cells, the UE comprising: a radio frequency (RF) unit configured to transmit or receive a radio signal; and a processor coupled to the RF unit, wherein the processor is configured to: receive M downlink subframes associated with an uplink subframe n in each of the two serving cells, and transmit an ACK/NACK response for the M downlink subframes received in each of the two serving cells by using one resource selected from four candidate resources in the uplink subframe n, wherein the two serving cells comprises a first serving cell and a second serving cell, wherein in the M downlink subframes received in the first serving cell, if a physical downlink shared channel (PDSCH) indicated by detecting a first physical downlink control channel (PDCCH) with a downlink assignment index (DAI) value equal to 1 or a second PDCCH with a DAI value equal to 2 is received, or if a first semi-persistent scheduling (SPS) release PDCCH with a DAI value equal to 1 or a second SPS release PDCCH with a DAI value equal to 2 is received, among the four candidate resources, a first resource is determined based on a first control channel element (CCE) used in transmission of the first PDCCH or the first SPS release PDCCH, and a second resource is determined based on a first CCE used in transmission of the second PDCCH or the second SPS release PDCCH.
 8. The UE of claim 7, wherein at least one downlink subframe among the M downlink subframes received in the first serving cell comprises a physical downlink control channel (PDCCH) in which a downlink grant is transmitted and a PDSCH corresponding to the PDCCH.
 9. The UE of claim 8, wherein the downlink grant includes a downlink assignment index (DAI) indicating an accumulative number of PDCCH with assigned PDSCH transmission.
 10. The UE of claim 7, wherein if a PDSCH, indicated by detection of a third PDCCH with a DAI value equal to 1 or a fourth PDCCH with a DAI value equal to 2 received in the M downlink subframes in the first serving cell, is received in the M downlink subframes in the second serving cell, a third resource among the four candidate resources is determined based on a first CCE used in transmission of the third PDCCH, and a fourth resource among the four candidate resources is determined based on a first CCE used in transmission of the fourth PDCCH.
 11. The UE of claim 7, wherein if a semi-persistent scheduling (SPS) physical downlink shared channel (PDSCH), which is a PDSCH having no corresponding physical downlink control channel (PDCCH), is received in the M downlink subframes received in the first serving cell, a first resource among the four candidate resources is one resource selected from four resources configured by using a higher layer signal, and the selected one resource is indicated by a PDCCH indicating activation of semi-persistent scheduling.
 12. The UE of claim 11, wherein, in the M downlink subframes received in the first serving cell, if a PDSCH indicated by detecting a first PDCCH with a DAI value equal to 1 or if a first SPS release PDCCH with a DAI value equal to 1 is received, a second resource among the four candidate resources is determined based on a first CCE used in transmission of the first PDCCH or the first SPS release PDCCH. 